The present invention relates generally to semiconductor device manufacturing and more specifically to techniques to make semiconductor devices which utilize wafer bonding.
Wafer bonding is a known technique to form two distinct single crystalline semiconductor materials within one final device. The two materials may have the same composition or be dissimilar materials. For example, two silicon wafers can be bonded together, or a germanium or gallium arsenide wafer may be bonded to a silicon wafer. In many instances, the two wafers are separated by a dielectric layer, which serves not only as an electrical isolation layer but also as an adhesion layer to form the bond.
In applications where devices are to be formed in both semiconductor wafers, it is desirable for one of the wafers to be sufficiently thin to facilitate device fabrication and formation of interconnects to the devices. However, starting off with a sufficiently thin wafer to bond to the receiving wafer is not practical because it has insufficient structural strength. One method to achieve a sufficiently thin bonded layer is to etch back one of the wafers after bonding. But this method can be undesirable in applications where a very thin resulting layer is required because such a large amount of the wafer material must be etched away and it is difficult to control the thickness uniformity of the remaining layer. An alternative technique has been developed which effectively transfers a thin layer of semiconductor material from a donor wafer onto the receiving wafer. The thin layer is defined in the donor wafer, for example, by performing a hydrogen implant to create a weakened region within the semiconductor wafer. The donor wafer and receiving wafer are bonded together, and a subsequent anneal process or cleaving process is used to separate the thin layer at the weakened region from the remainder of the donor wafer.
The above wafer bonding techniques are particularly useful in applications where the two wafers to be bonded are very planar, for example in forming silicon on insulator (SOI) on a silicon wafer. However, there are problems in applying the prior art wafer bonding techniques to applications in which the receiving substrate is not planar, for example where active devices and interconnect are already formed or are partially formed on the receiving wafer. Therefore, there is a need for an improved wafer bonding technique to accommodate non-planar wafers.
Moreover, it would be desirable for such a technique to also permit one of the semiconductor materials to be bonded and transferred only in selected areas to the other semiconductor material. While attempts have been made to accomplish selective bonding (e.g. by selectively performing a hydrogen implant in a silicon wafer in areas corresponding to where bonding was desired), such attempts fail to simultaneously address the problem of bonding to a non-planar receiving substrate.